Lecithin cholesterol acyltransferase (LCAT), the major enzyme which esterifies cholesterol present in plasma lipoproteins, plays a central role in HDL metabolism. Patients with LCAT deficiency may present with corneal opacities, anemia and renal disease as well as reduced plasma HDL-C and apoA-I concentrations. To evaluate the role that LCAT plays in reverse cholesterol transport and the development of atherosclerosis we have established a mouse model for human LCAT-deficiency by performing targeted disruption of the LCAT gene in mouse ES cells and back crossed the LCAT-KO mice into C57BL lines. Plasma LCAT activity in age-matched C57BL controls (n=30, LCAT act=42+/-5 nmol/h/ml) was decreased to <0.5 nmol/h/ml in homozygotes. Compared to control mice, homozygous LCAT-deficient mice had decreased cholesterol, cholesteryl ester, phospholipids, HDL-cholesterol and apoA-I. Analysis of plasma lipoproteins in homozygous LCAT-deficient mice by FPLC demonstrated severe reduction in HDL-cholesterol with the presence of smaller sized particles, as well as triglyceride-rich IDL/LDL. In response to a high fat, high cholesterol diet, homozygous LCAT-ko mice (n=9)had reduced plasma levels of cholesterol, cholesteryl esters, HDL-C and apoA-I. Homozygous LCAT-KO mice developed normochromic, normocytic anemia. Analysis of aortic atherosclerosis revealed significant reduction (p<0.05) in diet-induced aortic atherosclerosis in homozygous LCAT-KO compared to controls. Histologic and EM analysis of kidneys revealed mesangial cell proliferation and lipid deposits in homozygous LCAT-ko mice. The availability of a homozygous animal model for human LCAT deficiency will facilitate our understanding of the role that LCAT plays in the development of renal disease and atherosclerosis. Z01HL02059-02 Scavenger Receptor SR-BI facilitates the selective uptake of cholesteryl esters (CE) from HDL as well as native and modified apoB- containing lipoproteins. LCAT transgenic mice (L-Tg) accumulate large, CE-rich dysfunctional HDL leading to impaired reverse cholesterol transport (RCT). To elucidate the role of SR-BI in facilitating the uptake of CE from dysfunctional L-Tg HDL we studied the effect of adenovirus-mediated SR-BI (rSRBI-AdV)expression on the lipid profile, metabolism, and hepatic delivery of HDL-CE in control (C; n=5; LCAT act=32?4 nm/ml/h; lipids (mg/dl):TC=119?10, CE=95?7, FC=24?3, PL=196?12, TG=74?7, HDL-C=82?11) and (L-Tg; n=5; LCAT act = 3513?401;lipids:TC=224?6,CE=189?4,FC=35?2,PL=218?6,TG=76?5,HDL-C=178?8) mice. Four days after rAdV infusion, Western blotting revealed a similar (2.3X)increase in hepatic SR-BI expression in both study groups. SR-BI decreased baseline TC(-46% vs -32%), CE(-45% vs -30%), FC(-53% vs -33%), PL(-30% vs -22%)and HDL-C(-52% vs 30%)in L-Tg and C, respectively (p<0.05;all). FPLC showed that apoE-rich HDL1 and apoA- I/A-II HDL were decreased and the HDL-lipid composition was significantly altered with decreased dCE in L-Tg. The plasma clearance of autologous 3H-CE HDL was decreased(FCR=5.6?0.34 vs 3.85?0.23 d-1; p<0.004) and the hepatic delivery of 3H-CE HDL was similar (2 hr; 45% vs 50%),respectively in L-Tg and C (both increase vs baseline). In summary SR-BI: 1)decreased TC, CE and HDL-C in both L-Tg and C, 2)increased 3H-CE HDL clearance and hepatic HDL-CE delivery in L-Tg and C facilitating the selective uptake of CE from both native and dysfunctional HDL, 3)By increasing the hepatic uptake of increased CE generated by LCAT, overexpression of SR-BI partially corrects the reverse cholesterol transport defect associated with L-Tg HDL.